Your Lying Eyes

Dedicated to uncovering the truth that stands naked before your lying eyes.

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16 December 2008

Why Do Brothers and Sisters Make You Stupid?

That people in larger families have lower IQ's on average has been well known for quite awhile. But the GSS really shows this effect quite clearly. I looked at average WORDSUM (a 10 question vocabulary test) scores by number of siblings for whites. The number of reported siblings in the GSS astonishingly goes as high as 31, so I cut it off at 13 somewhat arbitrarily based on that being the last age whose WORDSUM average has a 95% confidence interval less than 1. Here's what we get along with the least-squares trend line:

To put things in perspective, the IQ difference between being an only child and being one of six is about 7 points and having 13 siblings 15 points. The most obvious reason for this is the Idiocracy effect - duller people have more children. We can look at that as well and compare the two - intelligence by number of children vs. number of siblings. For parents, I figured that some dumb parents with only a couple children might still be working on their brood, so I used ages 45 and up figuring that group would be pretty much done having all their kids.

They're not perfect matches - after all, they are measuring two entirely different sets of people, but they do track pretty close. There is that odd bump with parents where those with 2 children are smarter than those with 1, but otherwise it's a pretty steady descent towards dimness the bigger the family.

The odd thing with that bump though is that it's always there - if you only look at this decade, or only the 70's, or the late 80's or if you use blacks instead of whites - there's always a bump up in the parents' scores with one child to those with 2. Sometimes the bump lasts until the 4th child, but there's always that bump. But you never see it with the kids - only children always score better than others in these surveys, no matter how you slice it.

Anyway, I'm not claiming to have discovered anything new here, but the results were so stark I was rather amazed, plus even thinking about the new Obama administration is so unrelentingly depressing this stuff is a nice distraction.


Anonymous Anonymous said...

I liked the more colorful charts in the previous post. Keep'm coming. Fascinating stuff.

December 17, 2008 10:39 AM  
Anonymous Anonymous said...

HAHA I just "lol" at that comment.

Dad, you have been big with the charts. Is this a new discovery or something?

December 17, 2008 9:50 PM  
Blogger ziel said...

Alright, fine. I promise the next post will be 100% fact free!

December 17, 2008 10:57 PM  
Anonymous Anonymous said...

This is very interesting. Ive never heard the subject brought up (number of siblings and intelligence) before. I wonder what professional child psychologist and the like have to say about it and what do they attribute it to?

December 18, 2008 12:31 AM  
Blogger ziel said...

I think many would attribute it to the home environment, in that more children means less parental investment. I think there was a study recently showing the more older siblings you have, the dumber you are and this was claimed as evidence of environmental effect on IQ.

I recall that John Hawks brought up another interpretation of this. That parents of a dull child may be less inclined to have more, while precocious children may encourage more children. In other words, bad experience with the first children may discourage having more. For some reason I can't find that on his blog:

But it's also an unfortunate fact these days that smarter people have less children, and you can see from the second graph how closely the intelligence of parents by number of children track with people by number of siblings, so the simple assumption that children are smart/dumb because the parents are smart/dumb is not inconsistent with these results.

December 18, 2008 6:58 PM  
Blogger Black Sea said...

"But it's also an unfortunate fact these days that smarter people have less children, . . . "

Last night, I was watching "The First 48 Hours" a documentary show about homicide investigations. One victim, found in the middle of the road with his genitals on fire (after he'd been kiled, apparently), was a crack dealer with 22 . . . yes, 22, children. The narrator claimed that he'd been "forced to sell drugs to support his family."

I kind of doubt that he was in the business only to take care of his kids (there was never any mention of a wife or ex-wife). I'd also point out that before his demise, he'd managed to out-reproduce, by a factor of more than 10, your typical college educated white-collar drone.

Several of his children were interviewed on the show (one of his daughters was, for a time, the prime suspect) and they certainly seemed to confirm the theory that the larger the family, the dumber the kids.

December 19, 2008 12:28 AM  
Blogger ziel said...

Amazing. And think how much more damage these guys could do if they didn't end up dying young or going to jail so frequently.

I guess that explains the GSS responses to the # of siblings questions that are in the twenties.

December 19, 2008 5:29 AM  
Anonymous Anonymous said...

Z man- I called mom, mother of 7 (well, 6 now) and and told her about your discovery. There was a brief pause on the line before her reply of: oh, really.
I explained the reason why she and dad had so many kids: they were dull. I also brought up one of your previous posting points: having 7 was a status symbol. Age 84 mom had quite the chuckle. The reality in our our family is this: Child #1 and # 7 have the highest IQ's, where the lowest probably are with #'s 5 & 6.
Numbers 1-4 children cumualtively collected the most advanced degrees while at the same time smoked the most dope. (Number 3 & 4 never studied or took a test without igniting the bong. Their claim: it helped them concentrate)I was encouraged to smoke before sitting for the SAT's and declined. I did poorly. Numbers 2 & 7 obtained doctorates. 7 never touched drugs. Number 2 swore by them. I am going to chart my own findings and present it at the next family gathering. Thank you.

January 01, 2009 9:30 AM  
Blogger ziel said...

JF (Alt) - As they say, your mileage may vary. Note that your mom's first two were smart - that probably encouraged her to have more.

I must admit most large families I know are relatively smart - probably since it's awfully hard to afford a large family around here.

January 03, 2009 4:23 PM  

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